Flat-panel displays are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a display substrate to display images, graphics, or text. In a color display, each pixel includes light emitters that emit light of different colors, such as red, green, and blue. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the current. Displays using inorganic light emitting diodes (LEDs) are also in widespread use for outdoor signage and have been demonstrated in a 55-inch television.
Inorganic light-emitting diode displays using inorganic micro-LEDs on a display substrate are also known. Micro-LEDs can have an area less than 1 mm square, less than 100 microns square, or less than 50 microns square or have an area small enough that it is not visible to an unaided observer of the display at a designed viewing distance. U.S. Pat. No. 8,722,458 entitled Optical Systems Fabricated by Printing-Based Assembly teaches transferring light-emitting, light-sensing, or light-collecting semiconductor elements from a wafer substrate to a destination substrate.
Displays are typically controlled with either a passive-matrix (PM) control employing electronic circuitry external to the display substrate or an active-matrix (AM) control employing electronic circuitry formed directly on the display substrate and associated with each light-emitting element. Both OLED displays and LCDs using passive-matrix control and active-matrix control are available. An example of such an AM OLED display device is disclosed in U.S. Pat. No. 5,550,066.
Active-matrix circuits are commonly constructed with thin-film transistors (TFTs) in a semiconductor layer formed over a display substrate and employing a separate TFT circuit to control each light-emitting pixel in the display. The semiconductor layer is typically amorphous silicon or poly-crystalline silicon and is distributed over the entire flat-panel display substrate. The semiconductor layer is photolithographically processed to form electronic control elements, such as transistors and capacitors. Additional layers, for example insulating dielectric layers and conductive metal layers are provided, often by evaporation or sputtering, and photolithographically patterned to form electrical interconnections, or wires.
Typically, each display sub-pixel is controlled by one control element, and each control element includes at least one transistor. For example, in a simple active-matrix organic light-emitting diode (OLED) display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the luminance of the sub-pixel. Each OLED element employs an independent control electrode connected to the power transistor and a common electrode. In contrast, an LCD typically uses a single transistor to control each pixel. Control of the light-emitting elements is usually provided through a data signal line, a select signal line, a power connection and a ground connection. Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control.
In any application requiring many elements, it is important that each element is reliable to ensure good manufacturing yields and performance. Active-matrix control circuits, as well as the controlled element (e.g., a light emitter) are subject to failure. Because no manufacturing process is perfect, any large system can have defective elements. To ensure that large multi-element systems are reliably manufactured and operated, such systems can employ redundant elements. For example, displays are sometimes designed with redundant light emitters. U.S. Pat. No. 5,621,555 describes an LCD with redundant pixel electrodes and thin-film transistors to reduce defects. In another approach described in U.S. Pat. No. 6,577,367, an extra row or column of pixels is provided to replace any defective row or column.
An alternative approach to improving display yields uses additional, redundant light-emitting elements, for example two light emitters for every desired light emitter in the display. U.S. Pat. No. 8,766,970 discloses a pixel circuit with two sub-pixels and circuitry to determine whether a sub-pixel is to be enabled, for example if another sub-pixel is faulty. Similarly, U.S. Pat. No. 7,012,382 teaches an LED-based light system that includes a primary light source and at least one redundant light source. The primary light source is activated by itself and the performance of the light source is measured to determine whether or not to drive the redundant light source. The redundant light source is activated when the performance measurements indicate that a performance characteristic is not being met by the primary light source alone. The first light system can be activated in combination with the redundant light source once the decision is made to activate the redundant light source. U.S. Pat. No. 8,791,474 discloses redundant pairs of LED devices driven by a common transistor. WO 2014149864 describes separately controlled LED devices.
Thus, some prior-art designs use additional test or control circuits that require additional space over a substrate to switch between one element and a redundant element, if the one element is faulty. Other prior-art designs have a common controller or driver that can fail. Therefore, these arrangements do not address faults in the control circuits as well as in the light emitters and there remains a need for systems with improved reliability and simple structures.